Known SPP modulator devices exploit the high loss associated with surface plasmons for the construction of plasmon-polariton based modulators and switches. Generally, known plasmon-polariton based modulator and switch devices can be classified along two distinct architectures. The first architecture is based on the phenomenon of attenuated total reflection (ATR) and the second architecture is based on mode coupling between a dielectric waveguide and a nearby metal. Both architectures depend on the dissipation of optical power within an interacting metal structure.
ATR based devices depend upon the coupling of an optical beam, which is incident upon a dielectric-metal structure placed in optical proximity, to a surface plasmon-polariton mode supported by a metal structure. At a specific angle of incidence, which depends on the materials used and the particular geometry of the device, coupling to a plasmon mode is maximized and a drop in the power reflected from the metal surface is observed. ATR based modulators make use of this attenuated reflection phenomenon along with means for varying, electrically or otherwise, at least one of the optical parameters of one of the dielectrics bounding the metal structure in order to shift the angle of incidence where maximum coupling to plasmons occurs. Electrically shifting the angle of maximum coupling results in a modulation of the intensity of the reflected light. Examples of devices that are based on this architecture are disclosed in U.S. Pat. Nos. 5,155,617, 5,157,541, 5,075,796, 4,971,426, 4,948,225, 4,915,482, 4,451,123, 4,432,614, 4,249,796 and 5,625,729, the contents of which are incorporated herein by reference.
The ATR phenomenon may also be employed in an optical switch or bistable device, as disclosed in U.S. Pat. No. 4,583,818, the contents of which are incorporated herein by reference.
Mode coupling devices are based on the optical coupling of light propagating in a dielectric waveguide to a nearby metal film placed a certain distance away and in parallel with the dielectric waveguide. The coupling coefficient between the optical mode propagating in the waveguide and the plasmon-polariton mode supported by the nearby metal film is adjusted via the materials selected and the geometrical parameters of the device. Means are provided for varying, electrically or otherwise, at least one of the optical parameters of one of the dielectrics bounding the metal. Varying an optical parameter (the index of refraction, say) varies the coupling coefficient between the optical wave propagating in the dielectric waveguide and the lossy plasmon-polariton wave supported by the metal. This results in a modulation in the intensity of the light exiting the dielectric waveguide. Examples of such mode-coupling SPP modulators are disclosed in U.S. Pat. Nos. 5,067,788, 6,034,809, the contents of which are incorporated herein by reference. The paper ‘The proximity Effect of Conductors in Optical Waveguide Devices: coupling to Plasmon-Polariton Modes’ by P. Berini, SPIE Vol. 4111, pp. 60-68, July 2000’, further discusses the physical phenomenon underlying the operation of these devices.
These known modulation devices disadvantageously have limited optical bandwidth and, in the case of the ATR devices, are not readily coupled to input and output waveguides, such as optical fibers.
Modulators are known which do not use plasmon waveguide technologies, but are based upon voltage induced waveguiding, mode overlap changes, or mode extinction. For examples of these types of modulators, the reader is referred to ‘Voltage-Induced Optical Waveguide Modulator in Lithium Niobate’, by Jaeger et al., IEEE Journal of Quantum Electronics, Vol. 25, No. 4, 1989, pp. 720-728 and ‘Improved Mode Extinction Modulator Using a Ti-Indiffused LiNbO3 Channel Waveguide’, by Ashley et al., Applied Physics Letters, Vol, 45, No. 8, 1984 pp.840-842, the contents of which are incorporated herein by reference. In these types of modulators, the waveguide core is non-existent or weakly confining and the applied voltage either creates a core region where the index of refraction is raised enough to confine a mode or reduces the effective index of the mode below cut-off to induce radiation. These types of modulators have been demonstrated to suffer from at least one or all of the following limitations: high on state insertion loss, high drive voltage, and low off state extinction.
International patent applications Nos. WO 01/48521 and WO 03/001258 (Berini) disclose a modulator which can readily be coupled to a waveguide and which has more extended optical bandwidth than such known devices. The modulator comprises a waveguide structure formed by a thin strip of material having a relatively high free charge carrier density surrounded by material having a relatively low free charge carrier density, the strip having finite width and thickness with dimensions such that optical radiation having a wavelength in a predetermined range couples to the strip and propagates along the length of the strip as a plasmon-polariton wave. The surrounding material comprises two distinct portions with the strip extending between them. At least one of the two distinct portions has at least one variable electromagnetic property, and the device further comprises adjusting means for varying the value of that electromagnetic property so as to vary the characteristics of the waveguide structure and thereby the propagation characteristics of the plasmon-polariton wave. The adjusting means modulates an electric field in the at least one of the distinct portions. While such a modulator advantageously may provide a relatively high optical bandwidth and be readily coupled to a waveguide, such as an optical fiber or integrated optics waveguide channel, for it to be used effectively in optical communications it would be desirable for it to have a very low operating voltage, low insertion loss in the on state and deep extinction.